In the field of automobile industry, the tendency to size reduction and weight reduction has been strengthened. Further, front wheel front drive (FF) cars show a world-wide tendency to increase partly because of the demand for sufficient elbow room.
CVJ has been widely spreading also in Japan with model changes and the increase of independent rear suspension drive shafts (FR) cars. In FF cars, a fixed CVJ and a plunging CVJ are used in combination generally with the former outboard and the latter inboard. In FR cars, a plunging CVJ is often used both outboard and inboard.
A fixed CVJ tends to increase in temperature with an increase in angle, a reduction in size and weight or an increase in engine output. A plunging CVJ, which is used inboard, suffers from a temperature rise because the cooling effect during running hardly reaches and also because heat from differential gears is transmitted. A plunging CVJ is accompanied by reciprocal rolling and sliding on revolution and, as a result, resistance in the axial direction is apt to occur. The thus induced thrust has great influences on vibration of an automatic car body during idling, a shudder of a car body at the start and acceleration, and generation of beating noise or booming noise and vibration of a car body at a middle to high speed.
In order to reduce the induced thrust force, studies have been directed to improvements in structure and material of CVJ itself and improvements of lubricating grease to be applied to a joint.
High performance lubricating grease functions to suppress friction and wear of the sliding part of CVJ thereby serving for improvement in durability and reduction in vibration. Therefore, a high-temperature grease which exhibits improved extreme pressure properties and improved wear resistance and also withstands the above-mentioned elevated temperature of CVJ has been keenly demanded.
Under these circumstances, various lubricants for CVJ have been proposed to date. The most common of them is a grease composition comprising a purified mineral oil as a base oil and a lithium soap as a thickening agent. The grease of this kind usually contains additives for imparting extreme pressure properties, wear resistance, and friction inhibitory action, such as molybdenum disulfide, sulfurized fats and oils, and olefin sulfides. Recently, the use of a grease containing a calcium complex soap or urea which is more heat-resistant than a lithium soap as a thickening agent has been extending.
Typical examples of known grease compositions which seem relevant to that of the present invention will be mentioned below. U.S. Pat. No. 4,787,992 discloses a calcium soap-thickened front wheel drive grease, in which a thickening agent comprising a calcium soap or a calcium complex soap is used in combination with other additives, such as tricalcium phosphate and calcium carbonate, to impart extreme pressure properties to the base grease. U.S. Pat. No. 4,514,312 describes a grease composition comprising a urea grease having incorporated thereto an organomolybdenum compound and zinc dithiophosphate as additives. JP-A-4-304300 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a urea grease composition essentially containing prescribed amounts of a molybdenum dialkyldithiocarbamate sulfide, molybdenum disulfide, a zinc dithiophosphate compound, and one or more of oiliness improvers. JP-A-4-279698 discloses a grease composition for CVJ containing powdered boron nitride and an organozinc compound, such as zinc dithiophosphate.
However, the conventional grease involved any of disadvantages, such as insufficient performance in extreme pressure properties and wear resistance, tendency to induction of thrust force, and softening in high temperatures.